Plant Growth and Development: common doubts, answered
The questions students ask most often about Plant Growth and Development, each with a short answer. For the full chapter, read the Plant Growth and Development notes.
Growth and why it never stops in plants
Read this section in the notes →Why is plant growth called open growth?
Plant growth is open because meristems at the apices of roots and shoots keep producing new cells throughout the life of the plant. Animals, in contrast, grow to a certain stage and then stop. This capacity of unlimited growth is made possible by the presence of meristematic tissues, so plants add new organs repeatedly.
What is the difference between primary and secondary growth?
Primary growth is the increase in length of roots and shoots, brought about by the apical meristems. Secondary growth is the increase in girth, brought about by the lateral meristems, the vascular cambium and cork cambium. Secondary growth occurs in dicots and gymnosperms, but typically not in monocots.
What is the difference between growth and development?
Growth is an irreversible, permanent increase in size of a part or the whole organism, which needs energy. Development is a broader term that covers all changes in the life of an organism, from seed germination to senescence, and includes growth along with differentiation. So development equals growth plus differentiation.
Measuring growth
Read this section in the notes →Is a dry seed swelling in water an example of growth?
No. Swelling by absorbing water, as in imbibition, is reversible and does not need metabolic energy, whereas growth is irreversible and powered by metabolism. A piece of wood swelling in water, similarly, is only physical expansion. Growth should be judged by its permanent, metabolically driven increase.
Phases of growth
Read this section in the notes →What are the three phases of growth?
The three phases are the meristematic phase, in which cells divide at the tips of root and shoot, the elongation phase, in which cells just behind the tip enlarge and form vacuoles, and the maturation phase, in which cells reach their final size and differentiate. The root tip shows all three in a sequence.
Arithmetic and geometric growth
Read this section in the notes →What is the difference between arithmetic and geometric growth?
In arithmetic growth only one of the two daughter cells keeps dividing, so growth is linear, shown by Lt = L0 + rt, as in a root elongating at a constant rate. In geometric growth both daughter cells divide, giving an exponential increase, W1 = W0 e^rt, with a sigmoid curve when plotted over the entire period.
What are the three stages of the sigmoid growth curve?
The lag phase is the slow initial growth, the log or exponential phase is the period of fastest growth, and the stationary phase is when growth slows because nutrients or other factors become limiting. Together these give an S-shaped curve. It is typical of geometric growth of cells, organs and whole plants.
Absolute and relative growth rate
Read this section in the notes →What is the difference between absolute growth rate and relative growth rate?
Absolute growth rate is the total growth in a given time. Relative growth rate is the growth per unit time per unit of initial size. If two leaves gain the same area, the smaller leaf has the higher relative growth rate, since the gain is a bigger fraction of its starting size.
Conditions for growth
Read this section in the notes →What conditions does a plant need for growth?
Plant growth needs water, oxygen and nutrients as the basic requirements. Water gives turgidity for cell enlargement and is a medium for enzymes, oxygen supports respiration to release energy, and nutrients are needed for protoplasm and as an energy source. Temperature and light also affect growth, and each plant has an optimum range.
Differentiation, dedifferentiation and redifferentiation
Read this section in the notes →What are differentiation, dedifferentiation and redifferentiation?
Differentiation is the process by which cells derived from meristems mature to perform specific functions. Dedifferentiation is when living differentiated cells that have lost the ability to divide regain it, as in interfascicular cambium. Redifferentiation is when such dedifferentiated cells once more lose division capacity and mature to carry out specific functions.
Development and plasticity
Read this section in the notes →What is plasticity in plant development, and what is heterophylly?
Plasticity is the ability of plants to follow different pathways to give different structures in response to the environment or phases of life. Heterophylly is an example, with different leaf shapes on one plant, as in cotton, coriander and larkspur, which change with age, or in buttercup, where leaves differ in air and water.
Plant growth regulators and their discovery
Read this section in the notes →Who discovered auxin, gibberellin, cytokinin and ABA?
Auxin was identified by F.W. Went from oat coleoptile tips, after earlier work by the Darwins. Gibberellins were discovered by Kurosawa, who studied the bakanae disease of rice caused by Gibberella fujikuroi. Skoog and Miller found kinetin, a cytokinin. ABA was found by three groups as inhibitor-B, abscission II and dormin, and ethylene by Cousins.
Auxins
Read this section in the notes →What are the main functions of auxins?
Auxins help stem cuttings to root, maintain apical dominance, promote flowering in pineapple and induce parthenocarpy in tomato. They also control xylem differentiation, and the synthetic auxin 2,4-D is used as a herbicide against dicot weeds. IAA and IBA are natural auxins isolated from plants, whereas NAA and 2,4-D are synthetic, so do not call every auxin other than IAA synthetic.
Does 2,4-D kill all plants?
No. The synthetic auxin 2,4-D is a selective weedkiller that kills dicotyledonous weeds but does not harm mature monocotyledonous plants, so it is used to clear weeds from lawns and cereal crops. Plant growth regulators therefore have practical uses in agriculture.
What is apical dominance and which hormone controls it?
Apical dominance is the inhibition of lateral buds by the apical bud, and it is maintained by auxin produced in the apex. Cytokinins help overcome apical dominance and promote the growth of lateral shoots. Removing the apical bud, by pruning, therefore makes the plant bushy as the lateral buds begin to grow.
Gibberellins and cytokinins
Read this section in the notes →What are the uses of gibberellins?
Gibberellins increase the length of grape stalks, improve the shape of apples, delay senescence of fruits, speed up malting in the brewing industry, and increase sugarcane stem length, so yield rises. They also cause bolting, the sudden elongation of the stem, in rosette plants such as cabbage before flowering.
Is kinetin a natural cytokinin?
No. Kinetin was first obtained from autoclaved herring sperm DNA by Skoog and Miller and does not occur naturally in plants. Zeatin, isolated from corn kernels and also found in coconut milk, is a natural cytokinin. Cytokinins promote cytokinesis, delay leaf senescence and help produce new leaves and chloroplasts.
Ethylene and abscisic acid
Read this section in the notes →Why is ethylene called the fruit ripening hormone?
Ethylene is a gaseous hormone that speeds up the ripening of fruits and is linked with the rise in respiration, called the respiratory climacteric, during ripening. It also promotes elongation in deep-water rice, and the female flowers in cucumbers. Ethephon, which releases ethylene, is used to ripen tomatoes and apples.
Why is abscisic acid called the stress hormone?
ABA is called the stress hormone because it helps plants cope with adverse conditions, for example by closing stomata when water is short. It also promotes seed dormancy and inhibits seed germination. It acts as a general growth inhibitor and as an antagonist to gibberellins.
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